Ceramic materials having high strength and moldability are useful as tools and other articles which require a great deal of use and must do heavy work such as cutting tools.
For many technological applications it is desirable to fabricate ceramic parts with complex geometry. Since ceramic materials are inherently hard, shaping by machining methods after the part is fired is difficult and expensive. Therefore, it is desirable to form ceramic parts close to their final shape in order to minimize the amount of machining required on the final fired part. Two categories of methods are used to achieve complex shapes: green machining methods, and near-net shape forming methods. Green machining refers to shaping an unfired ceramic part using conventional machining methods. Near-net shape forming involves molding a slurry or paste containing ceramic powder to the desired shape. Examples of near-net shape forming processes are slip casting, injection molding and gelation-based casting methods.
Injection molding of ceramic parts occurs in a manner similar to injection molding of plastics. A granular precursor material composed of ceramic powder dispersed in a thermoplastic organic binder system is heated until it softens and is forced into a mold cavity under high pressure e.g., 30 MPa or higher. The organic binder is then removed and the compacted powder is sintered. While the process is easily automated, there are several drawbacks. Shrinkage of the thermoplastic binder can lead to internal defects in the molded part. Binder removal is slow and can be as long as several days. Binder removal can cause deformation or cracks in the final part. The high pressures and abrasive particles lead to rapid wear of the tooling.
Slip casting uses a porous mold to remove liquid from a slurry. As liquid is removed, the suspended ceramic particles consolidate, beginning at the mold surface. Since the liquid is transported from the liquid slurry through the cast layer into the mold, soluble species such as binder molecules tend to migrate resulting in their nonuniform distribution and gradients in particle packing density. It is a fairly slow process and is labor-intensive.
Gelation-based casting methods rely on a controllable transition from a liquid slurry to a semi-rigid solid once the slurry has been introduced into a mold. Advantages of gelation-based casting are that the low viscosity slurry easily takes the shape of the mold, binder content is low and can be easily removed by pyrolysis, a variety of mold materials can be used, gelation occurs without the removal of liquid so binder migration does not occur and capital costs are low since no special machinery is required.
Two gelation-based casting methods have been previously reported. Gel Casting refers to a method by which monomers and dimmers in a slurry are polymerized in situ, forming a strong gel structure. Thus, in U.S. Pat. No. 4,894,194 acrylamide monomers are mixed into a ceramic slurry. Gelation occurs when an initiator is added which polymerizes the monomers. This method has been used successfully for a wide range of powders. The main drawback of the process is that it uses acrylamides, which before polymerization are neurotoxins. Another drawback is that some mold materials have been reported to interfere with the gelation process.
Direct Coagulation Casting (DCC) involves the coagulation of electrostatically dispersed particles in the slurry. This is done by disrupting the electrostatic stabilization mechanism by altering the pH or the ionic strength of the system by means of enzyme-catalyzed reactions. The main drawback of this process is that no organic binders are involved, so the strength of the unfired casts is low. Difficulty may be encountered in handling the parts and they cannot be green machined.
Direct Coagulation Casting (DCC) has been initially described by Graule, Baader and Gauckler of the Swiss Federal Institute of Technology initially described in T. J. Graule, F. H. Baader and L. J. Gauckler, "Direct Coagulation Casting (DCC)--Principles of a New Green Shaping Technique," pp. 626-31 in Fifth International Symposium on Ceramic Materials and Components for Engines, edited by D. S. Yan, X. R. Fu and S. X. Shi, World Scientific, New Jersey, 1994. In this process an electrostatically stabilized suspension of particles is caused to coagulate by a pH shift or an increase in salt concentration which causes a lowering of the electrostatic repulsion between particles. The coagulation is driven by addition of an initiator which slowly changes the pH or ionic strength of the system.